Process control for industrial automation processes or industrial automation devices is often supervised and regulated by a process control system such as a closed loop control process. A traditional approach in the use of closed loop control is to measure a value of a process output and compare the measured value with a reference value. There are also other objectives of control loop control, including set-point regulation, tracking (time-varying reference path), path following (varying reference independent of time), disturbance attenuation etc. When used as an input for a in a feedback control loop, any difference between the measured sensor value and the reference signal is fed into the controller. The controller then in turn sends signals to the actuator so that the reference value is approached.
A control loop from the Prior Art is shown in FIG. 1. The Prior Art is shown to have a comparator 2, a control unit 4, an actuator 6 and a sensor 8. Sensor 8 measures a value for an output of the process, a dimension for example, and the measured value is sent to comparator 2. Comparator 2 compares the measured value with a reference value, sends the result to control unit 4. Control unit 4 issues a control signal to actuator 6 to reduce the difference between the reference value and the measured output value.
Model based control approaches typically use a mathematical or statistical model. An example of model based control may use one or more state equations, of which one may be of the form:
                    ⅆ        x                    ⅆ        t              =          f      ⁡              (                  x          ,          u                )                        y      =              g        ⁡                  (          x          )                      ⁢                
where x is the vector of all state variables including time derivatives to any order of x,    y is the output vector,    u is a vector of the inputs,    t is time,    f and g are functions representing the system.
Over time the difference between the value of the output measured by sensor 8 is expected to approach zero. Time delays and other constant feedback effects are not modelled in the above model, but may also occur between the actuator and the sensor. Such disturbances are not included in the above exemplary model but would be present in a real system.
The traditional closed loop feedback system comprises hard-wired communication links. A disadvantage with hard-wired communication links is that changes in position of any component in the closed loop, such as a sensor or actuator, usually requires a stop in production or an extensive shutdown, especially in the case of analogue wired connections, and/or digital wired connections. Alternatively, such changes have to be delayed until a process shutdown may be programmed. In addition, hard wiring may be both expensive to replace and sometimes technically challenging to replace.
The addition or removal of components in a control loop may demand interruptions in production or even a shutdown to implement when hard wired control lines are in use.
Wireless communications have been used within industrial systems and standards such as IEEE-802.11 have been shown to be sufficiently robust for many industrial environments. However, wireless communications are more subject to dynamic variations and disturbances than hard-wired or optical networks.